1. Technical Field
This invention relates to digital television (DTV) and, more particularly, to a modification of the existing Advanced Television Standards Committee (ATSC) broadcast standard to improve the speed at which the periodically transmitted training sequence is found after channel selection by a viewer.
2. Background Art
Digital television (DTV) signals in the USA are sent using the Advanced Television Standards Committee (ATSC) standard modulation system which is an eight level vestigial sideband (8-VSB) modulation with an offset and suppressed pilot signal. The television receiver must perform several complex steps to properly decode the incoming signal. One of these steps is to perform equalization on the incoming signal to reduce the effects of multipath distortion which might otherwise render the signal unusable. The ATSC standard signal periodically includes a training sequence specifically for the purpose of allowing channel equalization and multipath compensation.
One difficulty of using this signal for training is that distortion in the channel may make it difficult for the receiver to decode the incoming signal in order to find the training sequence. Knowing exactly when the training sequence was occurring would allow the receiver to more quickly perform equalization of the signal, and thereby reduce the amount of time between the viewer""s selection of an ATSC channel and the display of a DTV program.
The present invention provides a modification of the existing ATSC broadcast standard to improve the speed at which the periodically transmitted training sequence is found after channel selection by a viewer. The speed at which synchronization is achieved to a continuous wideband overlay signal used for equalization of the decoded signal is also improved.
The pilot signal is modulated slightly in a manner synchronized with transmission of the training sequences. The modulation of the pilot signal is small enough that it is ignored by existing ATSC receivers; that is, the modulation is backwards compatible. Receivers using this invention can detect when the training sequence is being transmitted by decoding this pilot signal modulation. Receivers equipped with this invention are able to find the training sequence more quickly than receivers without the invention. As a result, properly equipped receivers will provide a decoded image to a viewer more quickly after a channel change than a receiver not so equipped. The invention has the additional benefit of reducing the time required to synchronize to a continuous wideband overlay signal which has been provided for equalization. This may be done by varying the modulation slightly to correspond to the start of a particular phase portion of a periodic sequence such as an m-sequence used for the overlay. For example, the conversion might be that the amplitude modulation reaches a maximum at the start of the particular phase portion or a submultiple such as, for example, every fifth start of a particular phase portion.